Building yukkurarium is a difficult hobby because of the trouble to make a closed ecosystem that can sustain yukkuri life. But all that changed with the discovery of a special moss that can eat from bean paste.
This moss is on high demand so it's difficult to get.
Anon finally got the moss, which is essential to build a yukkurarium. The moss is a Happyness! tasted soft food for yukkuri. At the same time, the moss absorb moisture and get nutrition from paste, including yukkuri shit, which quickly ends the trouble with how to clean their excrements.
He got the KoReimu from her dying mother. He promised to try and make the baby (still on stalk) feel easy. The already badly beaten mother (not by Anon) don't care if she had to die.
KoReimu is injected with growth stopping formula so she doesn't overgrow the place and moss. Usually peoples would use mini-yukkuri, but Anon want newborn because they're still innocent and quickly accept the world around them.
In most case, he yukkuri living in the yukkurarium will die. They will feel stressed from loneliness and then getting weaker. As they grow weaker, they eat less and thus unable to balance out the moss' growth and ends up being consumed instead.
But maybe because the ground is her mother, this Reimu quickly adapts and always feel easy as she eats the moss. She awake in the morning, eat Mister Soft Soft, play alone, eat again, have afternoon nap, and sleep at night. That's why for long time, she can survive.
Anon is not sure if the mother is still alive or not, but he did put in her paste core together with the paste inside.
In the other hand, Mother Reimu's abusive husband came with a KoMarisa, demanding Anon to give her wife back. Anon killed the Marisa and mixed her paste with Reimu's leftover paste then planted the moss to them inside a box. In that box, he kept KoMarisa without growth stopper.
It's either KoMarisa will die because she overeat the moss, or she grow stressed and ends up getting eaten by moss.